It originated in Video Games (MMOs) like League of Legends or DoTa. It's a role that a player is assigned with the expectation to carry the team to victory. Other players on a team 'feed' the carry to get them strong enough to deal high attack damage to the opposing team and hopefully get them the victory.
GOT7 DID NOT INVENT THE WORD, Jackson explained where they got the term in one of their interviews.
Hard-carry is a Konglish internet slang used to describe a heroic performance from a specific person that gives a victory or such achievement to the whole team or works, which otherwise would have lost or failed. It was first used by online gamers when a specific gamer from a losing team play so good and singlehandedly carries the team into victory. Now it's widely used in similar situations in general sports or entertainment as well.
Cavs are losing. Now all we can look forward to is Lebron James' hard carry
The movie would have been boring without the main actor's hilarious acting. He hard-carried the movie.
For instance 3 slotted Phanto Assasin is much stronger than a 3 slotted Medusa, but when they are both 6 slotted Phantom Assasin will get rekt by Medusa.
We need to end this game before 40, they have hard carry.